Flyback converters are used as battery chargers and AC adapters that can supply a constant current to a heavy load and a constant voltage to a light load, for example as a battery becomes fully charged. Thus, flyback converters are said to operate in a constant current mode and in a constant voltage mode. Flyback converters can also operate in various modes of the switching cycles. For example, flyback converters operate in a discontinuous conduction mode (DCM), a critical conduction mode (CRM) or a continuous conduction mode (CCM). In discontinuous conduction mode, there is a time gap after all of the energy that was transferred to a secondary inductor has been released and before current again begins to ramp up through a primary inductor.
In the constant current mode, the current output by the flyback converter ideally should not exceed the specified current limit, despite changing operational conditions and inconsistent manufacturing processes. For example, the output current in the constant current mode should not exceed a predetermined current limit despite (i) variations with temperature of the base-emitter offset voltage of the inductor switch, (ii) propagation delays of current sense comparators and delays in turning off the inductor switch, or (iii) variations in the inductance of the inductors of the transformer of the converter.
The inductance of a typical inductor type can vary by ±20%. Considering this low precision of the conventional processes for manufacturing inductors, the variations in the inductance LP of the primary inductor can significantly divert the output current from the predetermined current limit.
There are various existing designs for flyback converters that attempt to generate an output current that does not exceed a specified current limit. FIG. 1 (prior art) illustrates an exemplary prior art flyback converter 10 that generates both constant output current and constant output voltage. Flyback converter 10 operates in discontinuous conduction mode. Flyback converter 10 includes a transformer 11, a transistor T1 12, a controller integrated circuit (IC) 13, a current sense resistor RCS 14 and a current set resistor RISET 15. Transformer 11 includes a primary inductor 16, a secondary inductor 17, and an auxiliary inductor 18. Transistor T1 12 acts as a switch to primary inductor 16. Feedback to regulate output current and voltage is received from a reflected voltage through transformer 11.
FIG. 2 (prior art) is a more detailed schematic diagram of controller IC 13. Controller IC 13 includes a first comparator 19, a second comparator 20, an OR gate 21, an RS latch 22, an oscillator 23, a driver 24, an error amplifier 25, a compensation network 26 and a constant voltage source 27. In addition, controller IC 13 includes a power pin (VDD), a ground pin (GND), a current set pin (ISET), an output pin (OUT), a current sense pin (CS) and a feedback pin (FB).
In operation, oscillator 23 initiates a turn-on pulse that sets RS latch 22. Thereupon, RS latch 22 outputs a digital high onto output lead Q, and driver 24 turns on transistor T1 12. When transistor T1 12 turns on, current ramps up linearly through primary inductor 15 at a rate defined by the primary inductance LP and the input voltage VIN. As the current flowing from the emitter of transistor T1 12 increases, the voltage across current sense resistor RCS 14 also increases. The voltage across current sense resistor RCS 14 is received on current sense pin CS and is compared by second comparator 20 to the voltage level VILIM. When the voltage across current sense resistor RCS 14 reaches the voltage level VILIM, second comparator 20 trips, and transistor T1 12 is turned off. Transistor T1 12 remains off until all of the energy that was stored in the transformer while current was ramping up through primary inductor 15 is transferred to the secondary side of flyback converter 10. At this point, the voltages across all of the inductors begin to fall toward zero until oscillator 23 outputs the next turn-on pulse, at which point transistor T1 12 turns on and the switching cycle repeats.
In the constant current mode, the output current of flyback converter 10 is limited by adjusting the output current such that the voltage across current sense resistor RCS 14 equals the predetermined voltage level VILIM. The voltage level VILIM is conventionally set, however, based on the following relationship between VILIM and the output current IOUT:
                                          I            OUT                    =                                    1              2                        ·                          L              P                        ·                          (                                                V                  ILIM                  2                                                  R                  CS                  2                                            )                        ·            Y            ·            η                          ,                            (        28        )            
where LP is the inductance of primary inductor 15, Y is a constant and η is the efficiency of flyback converter 10. The output current IOUT is set by adjusting the resistances of current sense resistor RCS 14 and current set resistor RISET 15 based on the inductance of primary inductor 15 as stated by the manufacturer. Equation 28 illustrates, however, that the actual output current IOUT will deviate from the desired set output current by the proportion by which the primary inductance LP deviates from the stated inductance.
A flyback converter is sought that can output a current at a predetermined limit that does not vary as a result of a variation in the inductance LP of the primary inductor. In addition, a method is sought for generating a power converter output current that has a magnitude that is independent of the primary inductance LP of the power converter.